Known gripping tools generally comprise a hydraulic or pneumatic jack permitting actuation of a gripping member such as a gripping arm which is drivable pivotably between an active gripping position on the piece and an inactive position. Such a tool can also comprise a guide which is introduced into a passage provided in the piece to be treated, which permits correctly centering the tool on the piece.
From FR-A-2 340 798, there is known a gripping tool whose gripping arm comes into abutment against the piece to be gripped by pivotably driving this gripping arm. This pivotal driving of the gripping arm between the release position and the gripping position is obtained by an articulated linkage mounted on a rod driven in linear displacement by a jack.
This pivotal movement of the gripping arm in this type of gripping tool has the drawback that, with certain pieces to be treated, there is obtained bad gripping, for example when the surface of the piece to be treated is not perfectly flat. In this case, the gripping arm is not always correctly positioned on the piece and does not guarantee good gripping. There exists moreover the risk of deformation of the piece.
Moreover, in the case of a gripping tool provided with a guide, the contact point of the gripping arm on the piece is relatively far from the centering point of the tool on said piece, which can give rise to a certain unbalance or require a more complicated structure of the tool.
There is known from WO-A-9635547, an indexing device for a piece through a hole in the piece and comprising a sleeve having an upper surface forming an abutment for a piece, a piston mounted in the sleeve to slide under the influence of drive means between a retracted position within the sleeve and an extended position in which it projects from the upper abutment surface of the sleeve by an end portion forming an indexing pin, a restraining jaw for the piece on the upper abutment surface of the sleeve, movably mounted on the centering pin and means to control the jaw comprising a slide mounted in an axial bore of the piston and having an end forming a cam. These control means are constituted by a slide mounted in an axial bore of the piston to slide under the influence of drive means between a first position in which it ensures the locking of the jaw in its active position by means of an active portion forming a cam driving in pivotal movement the gripping jaws toward said active position, and a second position in which it releases the locking of this jaw by controlling the pivoting of the gripping jaws toward the inactive position.
The drive means comprise among other things a first return spring disposed between the piston and the sleeve to return the piston to its extended position and a second return spring disposed between the sleeve and the slide to return the slide toward its unlocking position of the jaw.
The axial movements of the piston within the sleeve and of the slide in the piston are controlled by a jack. A principal drawback of this type of device is that, in the case of accident, for example upon a cutoff of the air supply, the jack no longer operates and thus, the second return spring blocks the device in the gripping position. Accordingly, in the gripping position, it is thus not possible to free the piece thus held.
Moreover, this structure is relatively complicated and the wear, particularly expansion of the springs, can give rise to correct positioning problems of the jaws with the passage of time.